TOPIC 14: Historic Climate Variability
THE OZONE LAYER
Some Initial Facts
* Ozone is a gaseous molecule which occurs naturally in the Earth's atmosphere. It is composed of three atoms of oxygen.
* The ozone layer, which contains anomalously high concentrations of ozone, occurs in the stratosphere 10-50 km above the Earth's surface. (Ozone also occurs near the Earth's surface as a result of pollution, but it breaks down before it can reach the stratosphere and be part of the ozone layer.)
* The stratospheric ozone layer absorbs short wavelength (ultra-violet) radiation that comes from the Sun.
* This absorption heats the stratosphere and causes global upper atmospheric air circulation (winds follow lines of latitude; eastward in the northern hemisphere, westward in the southern hemisphere).
* Without the ozone layer, there would be enhanced levels of skin cancer. Similar sorts of things might happen with loss of the Earth's magnetic field or a significant portion of the atmosphere. The magnetic field and atmosphere are buffers to high energy cosmic rays (protons and helium nuclei) that continually bombard the Earth. Without them, there would be an increased incidence of mutations and cancer.
Process of Ozone Formation
* Ozone forms at equatorial latitudes through the photodissociation of O2. The resultant ozone (O3) is stable for several weeks.
* The ozone in the equatorial stratosphere is transported toward the poles by stratospheric disturbances (essentially upper atmospheric hurricane-like storms).
* This leads to a latitudinal and seasonal distribution of ozone in the stratosphere. (A key feature to note is that the disturbances tend to increase the level of ozone at high latitudes seasonally above a base level. What is known as the polar ozone hole is a substantial decrease of the baseline. This cannot be caused by changes in the stratospheric disturbance pattern. It must be due to a process that removes ozone.)
How Polar Ozone Holes Form
* During the polar winter (little sunlight) season, high altitude cloud formations develop called polar stratospheric clouds (PSCs). The PSCs contain a high concentration of ice crystals which help to speed up a number of critical atmospheric chemical reactions. (Chemical reactions occur on the ice crystal surfaces making them essentially catalysts.)
* One of the chemical reactions that is enhanced by PSCs is the dissociation of HCL (hydrochloric acid - occurs with atmospheric water vapor) to form ClO (chlorine oxide). This is done by robbing ozone of one oxygen atom leaving regular atmospheric oxygen (O2).
* The key to this ozone loss is chlorine. It is argued that the chlorine comes from chlorofleurocarbons (CFCs) that are manufactured for use in aerosol cans.
* The ozone depletion has amounted to a net 6% loss for the winter season in the Arctic and about 50% loss for the winter season in the Antarctic.
Points of Disagreement
* Other sources of atmospheric chlorine could be volcanic gasses or biomass burning.
* There was no evidence for an increased level of ultraviolet radiation at the Earth's surface in the USA prior to the last 2-3 years. We are just now beginning to see a variety of biological problems at high altitude and/or high latitude which might be caused by ozone depletion.
* Unknown questions - What will it take to extend the ozone depletion to lower latitudes and more permanent loss?
Solution(s):
* If we can bet human contributions of chlorine out of the stratosphere, the stratospheric system of ozone generation and distribution will fix the observed ozone holes. The problem is that it may take 100 years or more to get rid of the stratospheric chlorine.
THE GREENHOUSE EFFECT
Some Initial Facts
* Carbon dioxide (CO2) is a normal component of the Earth's atmosphere. Its concentration has varied naturally from 100 to 250 ppm over the last few hundred thousand years as we oscillate into and out of glacial intervals.
* CO2 allows short wavelength solar radiation to pass through the atmosphere and heat the Earth. As the Earth re-radiates that heat back out as long wavelength radiation, CO2 absorbs that radiation and keeps the heat in the lower atmosphere. This is the same principle as a glass-enclosed greenhouse. The glass lets short wavelength radiation through into the greenhouse, but reflects long wavelength radiation (from inside the greenhouse) back into the greenhouse.
Natural CO2 variability
* CO2 is an integral part of the plant photosynthesis process. Plants absorb CO2 during the day as they generate energy, and they give off CO2 at night as they respire or use energy. A certain proportion of the CO2 is used to build new organic tissue so that there is a net negative flux of CO2 between plants and the atmosphere. This is later balanced after plants die and their organic material is decomposed. A similar pattern occurs in the oceans with plankton etc. But in the oceans, when plankton die, they may be deposited as carbonate rich sediments. This takes some CO2 out of the carbon cycle for much longer periods of time.
* There is also a long term natural variability due to changing climate. We know from direct measurement of the air trapped in ice (Greenland, Antarctica, etc.) that during glacial times the CO2 concentration is lower than during interglacial times. This is thought to be due primarily to increased plant productivity during warmer time.
* The highest level of CO2 before man started burning fossil fuels was about 280 PPM. This is about the highest value for the last few hundred thousand years.
Recent Changes in Global CO2 Level
* Within the last 100 years, the global CO2 level has increased to almost 350 PPM.
* This rise is thought to be due to anthropogenic causes (HUMANS). The most likely cause is the burning of fossil fuels - coal and oil - that has markedly increased since the industrial revolution of the 19th century. Changing land use patterns and deforestation may also be an important cause of the increased atmospheric CO2.
Implications of Global CO2 Rise
* We also have evidence that there is a slow but noticeable increase in global temperature over the last 100 years. This increase is attributed to the greenhouse effect.
* There is also some indication of a change in the global pattern of rainfall that may also be due to the greenhouse effect. The pattern is an increase in rainfall above 35° latitude and a decrease in equatorial regions.
* Geological evidence suggests CO2 levels much higher in geologic past. What was effect?
Points of Disagreement
* The temperature effect is still small and could be due to other natural aspects of climatic variability. Computer models predict much larger temperature rises that we have seen.
* Deforestation effect is not clear. Some models say that young forests remove CO2 more effectively than old forests, other models say no.